Since organic electroluminescent elements (which may hereinafter also be referred to as “elements” or “organic EL elements”) are capable of high-luminance light emitting using low voltage driving, they have been actively researched and developed. The organic electroluminescent elements have a pair of electrodes and an organic layer between the pair of electrodes, and utilize, for light emitting, energy of the exciton generated as a result of recombination of the electron injected from the cathode and the hole injected from the anode in the organic layer.
In recent years, high efficiency in organic electroluminescent elements is being advanced by using a phosphorescent material. However, in practical implementation, improvements are demanded from the viewpoints of a lowering of driving voltage and durability.
On the other hand, organic electroluminescent elements using, as a host material of the light emitting layer, a compound having a structure in which phenyl groups of triphenylamine are connected to each other and fused to from a carbazole ring or the like are known.
Patent Document 1 describes an organic electroluminescent element using, as a host material of the light emitting layer, a material having a structure in which benzene rings of a triphenylamine are mainly connected to each other via a methylene chain or the like and combining it with a phosphorescent material, and it can be read from Patent Document 1 that high efficiency, low voltage, and element durability are greatly enhanced. As for the means of connecting the benzene rings of a triphenylamine to each other, Patent Document 1 does not describe that a single bond is particularly excellent.
On the other hand, Patent Document 2 describes an organic electroluminescent element using, as a host material of the light emitting layer, a compound having a structure analogous to that in Patent Document 1 and combining it with a phosphorescent material and reports that when formed into an element capable of undergoing red phosphorescence emission, high efficiency and low voltage are achieved. Though Patent Document 2 describes various compounds in which benzene rings of a triphenylamine are connected to each other via a methylene chain or a single bond, it does not describe any compound in which benzene rings of a triphenylamine are connected to each other via two or more single bonds.
Patent Document 3 describes a compound having a structure in which two phenyl groups of triphenylamine are connected to each other and fused and describes that by using this compound as a host material of the light emitting layer, an organic electroluminescent element with good luminous efficiency and low driving voltage can be provided. Though Patent Document 3 describes some compounds having an indolocarbazole skeleton in which benzene rings of a triphenylamine are connected to each other via two single bonds, it does not describe any indolocarbazole having an oligoaromatic hydrocarbon ring as a substituent.